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Abstract:

This disclosure provides articles that include functionalized nanoscale
fibers and methods for functionalizing nanoscale fibers. The
functionalized nanoscale fibers may be made by oxidizing a network of
nanoscale fibers, grafting one or more molecules or polymers to the
oxidized nanoscale fibers, and cross-linking at least a portion of the
molecules or polymers grafted to the oxidized nanoscale fibers. The
functionalized nanoscale fibers may be used to make articles.

Claims:

1. A method for functionalizing a network of nanoscale fibers comprising:
contacting the network of nanoscale fibers with an acid or an oxidant,
effective to oxidize at least a portion of the nanoscale fibers;
contacting the network of nanoscale fibers with a conjugated polymer to
graft the conjugated polymer to at least a portion of the oxidized
nanoscale fibers; and cross-linking at least a portion of the conjugated
polymer grafted to the at least a portion of the nanoscale fibers by
irradiating the network of nanoscale fibers with an effective amount of
radiation, or by contacting the network of nanoscale fibers with a
chemical agent comprising two or more reactive functional groups.

12. The method of claim 1, wherein the crosslinking is by irradiating and
the radiation is UV radiation.

13. A method for functionalizing a network of nanoscale fibers
comprising: contacting the network of nanoscale fibers with an acid or an
oxidant, effective to oxidize at least a portion of the nanoscale fibers;
contacting the network of nanoscale fibers with a conjugated polymer to
graft the conjugated polymer to at least a portion of the oxidized
nanoscale fibers; and irradiating the network of nanoscale fibers with an
amount of radiation effective to cross-link at least a portion of the
conjugated polymer grafted to the at least a portion of the nanoscale
fibers.

14. An article of manufacture comprising: a network of nanoscale fibers,
wherein at least a portion of the nanoscale fibers are cross-linked by a
conjugated polymer or other molecule.

15. The article of claim. 14, wherein the conjugated polymer or other
molecule comprises a polydiacetylene.

16. The article of claim 14, wherein the network of nanoscale fibers has
an electrical conductivity of at least 6200 S cm.sup.-1.

[0002] This disclosure relates generally to nanoscale fibers, and more
particularly to methods for functionalizing carbon nanotubes or other
nanoscale fibers to increase or improve the properties of the nanoscale
fibers in buckypapers.

[0003] Since being discovered in 1991, carbon nanotubes (CNTs) have been
the subject of many studies due to their unique mechanical and electronic
properties. Recently, CNT macroscopic assemblies, such as thin films of
nanotube networks or buckypapers (BPs) and nanotube fibers, have drawn
much attention due to the potential to utilize the characteristic
properties of individual CNTs in macroscopic scale samples and products.
In particular, CNT assemblies may be used to fabricate organic electronic
devices, hybrid solar cells, super capacitors, transparent electrodes,
chemical sensors, field emission displays, artificial muscles,
high-surface-area electrodes, and high-performance nanotube-reinforced
composites.

[0004] However, the actual performance of macroscopic assemblies of CNTs,
such as electrical conductivity and mechanical strength, have not been as
high as expected. For example, most nanotube thin films and BPs have
electric conductivity values ranging from 10-1000 Scm-1, which is
much lower than that of an individual CNT (10,000-30,000 Scm-1 or
higher). The lower conductivities observed for single-walled carbon
nanotube (SWCNT) films may be due to the lack of alignment and short
nanotube lengths, resulting in high contact resistances and Schottky
barriers at inter-tube junctions. Bulk resistance of CNT network films
may be dominated by contact resistance among nanotubes or their ropes in
BP networks, which are one or two orders lower than the intrinsic
conductivities of individual nanotubes. Several models have been
developed to explore the effects of length, diameter and chirality on
contact-resistance-dominated electrical properties of CNT networks.

[0005] The introduction of chemical covalent bonding and charge moieties
between CNTs may enhance both the electric conduction and mechanical
properties as compared to those CNTs assembled by weak van der Waals
interactions. Chemical modification treatments of CNTs, such as acid
treatment, oxidation, and plasma etching have been reported to produce
functional groups (e.g., carboxylic acid, quinone, phenol, ester, amide
and zwitterions) on oxidized SWCNTs at their end caps and at defect sites
on their surface. Oxidations can also occur during SWCNT purification by
HNO3 oxidization when HNO3 is used to remove surfactant at CNT
junctions. These functional groups may enhance CNT interactions and
charge-carrying capability.

[0006] The conductivity of acid-treated CNT film conductivity can be
enhanced, resulting in the addition of charge carriers either in the form
of p-type or n-type doping. Acid doping methods convert the
semiconducting CNTs into metallic materials through effective tuning of
the nanotubes' Fermi level by either changing the conduction or valance
bands with electron doping or hole doping, respectively. Previously, CNT
Fermi levels have been tuned by chemical treatment, thereby increasing
the intrinsic conductivity of the SWCNTs while decreasing the inter-tube
resistance of the semiconducting-metallic junction through mitigation of
Schottky barrier. In addition, SWCNT films have showed relatively high
conductivity after being treated with strong acids such as HNO3. It
also has been demonstrated that the amount of p-dope SWCNTs can be
significantly increased by adjustment of the Fermi level of the valance
bands using a HNO3 treatment. For instance, Bower et al. observed
intercalation of HNO3 within a SWCNT network (Bower, C. et al. CHEM.
PHYS. LETT. 1998, 288, 481-486). Yu and Brus have showed that the
tangential mode of metallic SWCNTs in Raman scattering measurements
depends on the exposure to a HNO3 oxidization reaction (Yu, Z. and
Brus, L. E., J. PHYS. CHEM. A. 2000, 104, 10995-10999). Those findings
are attributed to charge transfer doping by acid treatments.

[0007] However, the doping effects of HNO3 have been shown to be
reversible, leading to questions regarding the overall stability of the
enhancements of doped films for real engineering applications. In device
and composite fabrication processes, doped CNT films of chemical
treatments may go through polymeric resin impregnation and curing, metal
deposition, and device encapsulation processes, for example. These steps
may involve exposures to various solvents and open air, as well as
elevated temperature conditions. Furthermore, actual device operation may
also need the doped CNT films to work in open air and elevated
temperatures. Therefore, doping stability is an issue for those
treatments. Previously, researchers have used a polymer coating layer to
protect doped CNTs and demonstrated an improved stability. Hence, it is
important to have CNT films with high electrical conductive properties
and doping stability when exposure to open air and/or elevated
temperature conditions are required or expected.

[0008] Chemical polymerization of CNTs with carboxylic groups by a
condensation reaction has been previously described. Ester bonds can be
synthesized by a dehydration condensation reaction of a carboxylic group
with a hydroxyl group in the presence of a
dehydration-condensation-coupling agent. The ester structures can be
further grafted onto a CNT surface by employing a dehydration
condensation reaction called esterification. This reaction is also useful
in grafting chemical molecules onto a CNT surface to conduct further
derivative reactions to realize cross-linked nanotubes, such as
polymerization reaction by using UV irradiation. The local coalescence
and cross-link polymerization of CNTs using chemical treatment with
thermal heating or UV irradiation resulted in the generation of vacancies
on the nanotubes, which can improve carbon nanotube electrical property,
and also improved the mechanical property. Irradiation cross-linked
nanotubes have also been reported for improved electrical and mechanical
properties of CNT networks. Recently, it also has been reported that
thiol-functionalized multi-walled carbon nanotube (MWCNT) was used to
facilitate CNT cross-linking. These CNT cross-links usually have high
doping stability due to covalent bonding; however, the lack of designated
conductive paths for charge-transfer improvement leads to only marginal
improvement in electrical conductivity.

[0009] Therefore, it would therefore be desirable to develop CNT networks
with high doping stability and designated charge-transfer paths to
overcome contact resistances.

BRIEF SUMMARY

[0010] In one aspect, methods are provided for functionalizing nanoscale
fibers. In one embodiment, the method generally comprises oxidizing
nanoscale fibers, grafting a molecule to the oxidized nanoscale fibers,
and cross-linking at least a portion of the grafted molecules. In other
embodiments, the method generally comprises oxidizing nanoscale fibers,
grafting polymers to the oxidized nanoscale fibers, and cross-linking at
least a portion of the grafted polymers. In certain embodiments, the
nanoscale fibers may be cross-linked by irradiation, or by contacting the
nanoscale fibers with a chemical agent having two or more reactive
functional groups.

[0011] In another aspect, articles of manufacture are provided which
comprise functionalized nanoscale fibers. The functionalized nanoscale
fibers may be functionalized by a method described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a schematic diagram for 1,4-addition polymerization
reaction of diacetylenes.

[0013] FIG. 2 is a schematic illustration of a CNT conjunctional
cross-linking process during ultra-violet (UV) irradiation, according to
an embodiment described herein.

[0018] FIG. 7 is a depiction of BP Resistance Variation/% for (a) MWCNT BP
versus time in air for CCL-MWCNT-BP (black square) and acid-treated MWCNT
BP (hollow square), and (b) BP resistance of SWCNT BP versus time in air
for CCL-SWCNT-BP (black square) and acid-treated SWCNT BP (hollow
square).

[0021] Methods have been developed to functionalize nanoscale fibers,
networks of nanoscale fibers, and nanoscale fibers in nanoscale fiber
films.

[0022] In one aspect, a method is provided to conjugationally cross-link
nanoscale fibers (e.g., CNTs) to achieve high electrical conductivity and
doping stability in CNT networks or thin films. In one embodiment, CNTs
are contacted with HNO3 and then a polydiacetylene (PDA) molecule to
create CNT cross-links with higher charge transfer capability. BPs
produced by such a method may have greater electrical conductivity,
doping stability, and/or mechanical properties.

[0023] The conjugational cross-linked BPs (CCL-BPs) produced by the
methods described herein may demonstrate high electrical conductivity;
for example, up to 6200 Scm-1, which is more than one order greater
than the electrical conductivity of pristine BP. Without being bound by a
particular theory, the mechanism of the electrical conductivity increase
is believed to be the increasing inter-tube electron transport
capability. The conjugational cross-links may provide effective
conductive paths to increase the mobility of electrons among individual
nanotubes. Unlike other chemical doping methods, some CCL-BP samples
advantageously have a doping stability of over 300 hrs in an ambient
atmosphere, and are generally resistant to degradation at elevated
temperatures.

[0024] In addition, the cross-links may improve the mechanical properties
of the BP materials. While not wishing to be bound by any particular
theory, these improvements may be the result of effective and stable
conjugational cross-linking of CNTs, which can improve BP electrical
conductivity, doping stability, and/or mechanical properties for their
potential use in engineering applications of macroscopic assemblies or
networks of CNTs.

Functionalizing Nanoscale Fibers

[0025] Methods for functionalizing a network of nanoscale fibers are
provided. In one embodiment, the method comprises contacting the network
of nanoscale fibers with an acid, contacting the network of nanoscale
fibers with a conjugated polymer to graft the conjugated polymer to at
least a portion of the nanoscale fibers, and irradiating the nanoscale
fiber film. The radiation, e.g. UV, cross-links at least a portion of the
conjugated polymer grafted to the nanoscale fibers. In another
embodiment, a chemical agent with two, three, or more than three,
reactive functional groups may be used to react with other agents to form
cross-link structures. In some embodiments, the radiation reaction is
preferred for PDA-type molecule curing due to its high efficiency.

[0026] Examples of suitable acids for use in the method include nitric
acid, sulfuric acid, and hydrogen chloride. Other acids and oxidants,
such as m-chloroperoxybenzoic acid and benzoyl peroxide, may also be
used.

Conjugated Polymers

[0027] In certain embodiments, the conjugated polymer is selected from
polydiacetylenes (e.g., 10,12-pentacosadiyn-1-OL (PCDO)).
Polydiacetylenes (PDAs) are a family of highly π-conjugated polymers
that have unique characteristics associated with their ability to
self-assemble. An example of a PDA is shown in FIG. 1. The ene-yne
backbone of PDA derivatives leads to optical and electrical properties
associated with extensively delocalized π-electron networks and
intrinsic conformational restrictions.

The Carbon Nanotubes

[0028] As used herein, the terms "carbon nanotube" and the shorthand
"nanotube" refer to carbon fullerene, a synthetic graphite, which
typically has a molecular weight between about 840 and greater than 10
million. Carbon nanotubes are commercially available, for example, from
Carbon Nanotechnologies, Inc. (Houston, Tex. USA), SouthWest
NanoTechnologies, Inc. (Norman, Okla. USA), or Nanocomp Technologies,
Inc. (Concord, N.H.) or can be made using techniques known in the art.

[0029] In one embodiment, the buckypaper is a thin film (approximately 20
μm) of nanotube networks, which can be utilized in various products,
such as composites, electronic devices and sensors. Buckypapers or thin
films may be made through the dispersion of nanotubes in suspension
followed by a filtration or evaporation process, stretching or pushing
synthesized nanotube "forests" to form sheets or strips, and the
consolidation of syntheses nanotube aerogels to form film membranes.

[0030] The functionalized nanoscale fiber films may be used to fabricate
highly conductive and stable nanoscale fiber sheet materials for both
immediate and near future Micro Electra Mechanical Systems (MEMS)
engineering, such as sensors, transistors, electrodes, actuators, fibers
and composite applications requiring high conductivity and mechanical
properties and thermal stability properties.

[0032] The methods and compositions can be further understood with the
following non-limiting examples.

Example 1

[0033] This example demonstrates the improved electrical conductivity and
high doping stability of BP resulting from conjugationally cross-linking
carbon nanotubes in the BP via chemical functionalization with ene-yne
backbone molecules. Thin, randomly oriented and aligned nanotube sheets
of millimeter-long multi-walled carbon nanotubes (MWCNT) manufactured by
Nanocomp Technologies, Inc. (Concord, N.H.) were used to produce
cross-linked samples. The aligned MWCNT had a small alignment degree
(<20% alignment degree by Raman spectrum measurement). These BP sheets
were mechanically strong, with a breaking strength of about 100 MPa, and
displayed high electrical conductivity (about 400 S cm-1). The
SWCNTs used in this example were produced by Carbon Nanotechnologies,
Inc. (CNI, Houston, Tex.). 10,12-pentacosadiyn-1-OL, (PCDO) material, one
of the commercially available PDA molecules, and nitric acid was
purchased from Sigma-Aldrich. The PCDO was dissolved in THF solvent, The
concentration of PCDO is higher than 1 nM and the solution was kept in a
dark vial to avoid undesired reactions with light. All the materials were
used as received.

Preparation of SWCNT Buckypaper

[0034] The SWCNT networks were prepared by a dispersion and filtration
process. First, the SWCNT powders were ground with a few drops of water
using a mortar and pestle. Then a bath sonication process (Sonicator
3000, Misonix, Inc.), was used for one hour to prepare a CNT suspension
with the aid of an aqueous Triton X-100 surfactant. The suspension
usually has 40 mg/L nanotube concentration and 400 mg/L surfactant
content. The suspension was filtered through a PTFE membrane (pore size
of 0.45 μm) under a 29 in Hg vacuum to produce randomly dispersed BP
samples having a 10-20 μm thickness. The samples were washed
repeatedly with distilled water and isopropanol to remove the surfactant.
The BPs were annealed at 550° C. in argon gas for 4 hours to burn
off the impurities and residual surfactant from the samples. The BP
sheets had a breaking strength of about 15 MPa and an electrical
conductivity of about 150 Scm-1.

Preparation of Conjugationally Cross-Linked CNT Films

[0035] Chemical functionalization of the BPs using a 12 M nitric acid
treatment was performed by immersing the BPs into the acid solution for 8
hrs. A doping time of 8 hrs was used because previous tests revealed no
significant difference in the Raman spectra of films processed with
immersion times increased from 10 hrs to 30 hrs. The treated films were
washed and dried in air. Subsequently, the films were baked in an oven at
50° C. to further remove residues. The acid-treated films were
immersed into the THF solution containing PCDO molecules for 2 hrs. Thus,
the functionalized BPs with carboxylic acid groups reacted with PCDO to
carry out esterification reactions, as shown in FIG. 2. After the
esterification process, the films were washed with THF, and then blown
dry. Esterified BP was subsequently treated with UV irradiation at the
wavelength of 365 nm with a sample-source distance of approximately 2 cm
in a nitrogen-purged dark chamber for 90 minutes. After polymerization
via a 1,4-addition reaction, the CCL-BP samples were rinsed with THF and
blown dry with a stream of nitrogen. The neighboring diactylenes (DAs)
were polymerized via a 1,4 addition mechanism by UV irradiation without
the need for chemical initiators or catalyst.

[0036] The conductivities of the BP samples were measured using a
conventional four-probe method. A Keithley 6221 meter was used as a
current source and a Keithley 2182 was used as a nano-voltmeter to obtain
characteristics of current-voltage curves, and a Labview program was used
to obtain a simultaneous voltage reading during current flow.

[0037] Thermal analysis of BP was performed using a thermogravimetry
analysis (TGA; TA Instruments Q800). All TGA measurements were carried
out under a nitrogen atmosphere flushed at 20 ml min-1 and under a
heating rate of 20° C. min-1 from 50° C. to
1000° C.

[0038] The mechanical properties of pristine and cross-linked BP samples
were tested using a Dynamic Mechanical Analysis machine (DMA Q800, TA
Instruments) under controlled forced mode with stress-strain sweeping of
0.5 N min-1 from 0 to 18 N. All BP samples had a dimension of 20
mm×3 mm.

[0040] To deter mine the effects of acid treatment and cross-linking on
the electrical stability of the BP samples, resistivity vs. air exposure
time relationships were monitored. To measure the electrical stability
property under thermal loading, BP samples were placed on a hot plate and
heated to given temperatures, and their resistances were measured.

Effect of Cross-Linking on Morphology, Raman and IR Spectra

[0041] The surface morphology of the MWCNT sheets before and after
cross-linking is shown in FIG. 3. The nanotube ropes can be seen on the
surface of the pristine films, and the nanotubes were randomly oriented
for both random and aligned samples due to limited alignment degree. The
surface morphology after cross-linking was changed. Most nanotube ropes
were wrapped up with the polymers due to the chemical cross-links.

[0042] The Raman D-band (˜1300 cm-1) to G-band (˜1590
cm-1) intensity ratio (D/G ratio) was a good indication to confirm
electronic structure changes of CNTs due to chemical functionalization.
FIG. 4 shows the D and G band ranges of the Raman spectra of MWCNT and
SWCNT samples before and after cross-linking processes. The cross-links
introduced more SP3 hybrids on the functionalized CNTs and the
disorder band (1300 cm-1) became much larger as compared to the
pristine carbon nanotubes. FIG. 4a shows that the D/G ratios of MWCNT BP
before and after the cross-links were 0.18 and 0.85, respectively. In
case of the SWCNT BP, there was a slight decrease in the intensity of the
tangential vibration of the G band at 1590 cm-1 with an increase of
broad D band at around 1300 cm-1, as shown in FIG. 4b, due to
possible degradation of SWCNT stiffness.

[0043] The presence of functional groups on the samples was identified
using IR spectroscopy with a reflection method. The IR spectra of the
pristine MWCNT BP, acid-treated MWCNT-BP, and cross-linked MWCNT-BP
through the estherification reaction are shown in FIG. 5. In the case of
acid-treated MWCNT BP (FIG. 5; middle), the small band at 1704 cm-1
was assigned to the stretching vibration of the C═O carboxylic acid
and carbonyl, while the bands at 3500 cm-1 correspond to the
stretching vibration of the O--H carboxyl, hydroxyl and phenolic groups.
On the other hand, after the esterification procedure (FIG. 5; bottom),
the peaks at 1100 to 1250 cm-1 reflect the presence of stretching
vibration modes of --C--O--C in the ester. The 1770 cm-1 peak
correlates with the stretching vibration of the C═O moiety in ester
groups. Based on the IR spectrum analysis it can be proposed that the
carboxylic acid and hydroxyl groups on the nanotube surface were created
through acid treatment and then converted into ester bonds with the
subsequently introduced PCDO molecule, and finally formed ester
structures by an esterification reaction, as shown in FIG. 2. Thus,
conjugational ester bonds in CCL-MWCNT-BP would provide cross-linking and
electrons or charge transfer between nanotubes.

Thermogravimetric Analysis (TGA).

[0044] TGA was employed to investigate thermal stability and PCDO
concentration of the samples before and after cross-link
functionalization. FIG. 6a compares the TGA profiles of pristine MWCNT BP
(top solid line, bottom dashed line) and cross-linked BP (top solid line,
bottom dashed line). The TGA curve of pristine MWCNT BP shows one
degradation stage before the final decomposition of the MWCNT BR The TGA
curve of CCL-MWCNT-BP shows three main degradation stages before the
final decomposition of the CCL-MWCNT-BP. The first weight loss region,
with about 4 wt. % loss of initial weight around 100-250° C., was
due to the evaporation of water molecules or monomer molecules. It was
believed that some uncross-linked monomers would absorb on the BP
surface. The second weight loss region, with about 17.2 wt. % loss of
initial weight around 250-560° C., was due to decomposition of
cross-linked molecules. The differential TGA curve shows the CCL-MWCNT-BP
having one peak at 400° C., which can be considered the
decomposition temperature of cross-linked PCDO molecules. Hence, PCDO is
about 18.3 wt. % in the CCL-MWCNT-BP sample.

[0045] FIG. 6b shows the TGA curves for pristine SWCNT (top solid line,
bottom dashed line) and cross-linked BP (top solid line, bottom dashed
line) samples. Decomposition occurred in two distinct steps for unreacted
monomers and cross-linked PCDO molecules of cross-linked samples. The
first decomposition was at around 100-350° C., probably
representing the cleavage of unreacted cross-linked monomer. A second
decomposition step was observed at around 350-600° C. for the
cross-linked molecules. The differential TGA curve shows one peak for the
CCL-SWCNT-BP at 490° C., which can be related to the decomposition
temperature of cross-linked PCDO molecules within the BP. Here, the PCDO
concentration was 24.0 wt. % of the CCL-SWCNT-BP. Much denser
cross-linking networks may be formed on SWCNT-BP due to the large surface
area and more reactive surface of SWCNT-BP samples. Therefore,
CCL-SWCNT-BP possesses a higher decomposition temperature than
CCL-MWCNT-BP.

[0047] The conductivity for these three types of pristine and acid-treated
BPs was less than 2400 Scm-1. It has been shown that acid treatments
could enable Fermi-level shifting into the van Hove singularity region of
metallic CNTs, resulting in a substantial increase in the density of
states at the Fermi level. Hence, electrical conductivity values of all
three BPs increased after the acid treatment. The electric conductivity
values of all three CCL-CNT-BP samples were further increased to one and
half times higher than that of the acid-treated BPs. This indicates that
the conjugational cross-links of CNTs could have a conjugation system for
electron transport to further increase electrical conductivity.
Particularly, for the aligned MWCNT BP sample, conductivity increased by
about 11 fold from 600 to 6,200 Scm-1 as compared to the pristine BP
at room temperature. Such a large conductivity increase was caused by the
formation of conjugation of ene-yne backbone in the cross-links, thereby
providing effective electron transfer paths within the CNT networks. But
the SWCNT BP improvement of conductivity is not as much as the
improvement in the MWCNT samples due to possible nanotube structure
damage, which can significantly degrade SWCNT's intrinsic conductivity.

Effect of Conjugational cross-link functionalization on conductivity
stability Conjugational cross-link structures, which eliminate or reduce
the number of unpolymerized molecules, should have high conductivity
stability. The effect of the cross-links in the BPs on the relationship
between electrical conductivity stability and open-air exposure time is
presented in FIG. 7. The MWCNT BPs with the nitric acid treatment showed
a resistance increase of 23% after being exposed to open air for 300 hrs,
while the CCL-MWCNT-BP showed no observable resistance changes under the
same conditions. For the SWCNT-BPs with the nitric acid treatment, a 25%
increase in resistance is shown after 200 hrs. Similarly, the
CCL-SWCNT-BP samples only had less than a 5% resistance increase after
220 hrs, as shown in FIG. 7b. The resistance change of the
nitric-acid-treated BP was proved to be easily reversible and degradable
due to conductivity depending on the mobile HNO3 and NOx
residues intercalation within the network. In contrast, introducing a
covalent bond with conjugated molecules to link individual carbon
nanotubes eliminated mobile moieties. Hence, CCL-BP conductivity was very
stable as compared to the acid-treated samples due to the designated
conjugational cross-links providing stable and permanent electrical
conducting paths through CNTs network.

Thermal Stability of Conjugational Cross-Linked BPs

[0048] The thermal stability of the electrical conductivity of CCL-BPs was
tested, since they may be used in elevated temperature environments. The
results of the thermal stability experiments are shown in FIG. 8.
Resistance variations of both acid treated BP and CCL-BP samples for
temperatures ranging from 20 to 150° C. were measured. As shown in
FIG. 8a, the electrical resistance of nitric-acid-treated MWCNT-BP
increased with the increase of temperature. Previous research indicates
that intercalated HNO3 and nitrogen oxide immediately desorbs from
CNT surface under thermal annealing at temperature greater than
320° C. This effect was observed at temperatures lower than
100° C., in which electrical resistance increased by up to 100%.
In contrast, the CCL-MWCNT-BP showed no change in electrical resistance
at temperatures up to 150° C. Thus, the CCL-BP showed the thermal
stability after exposure to elevated temperatures.

Mechanical Property Improvement

[0049] The tensile stress-strain curves of the acid-treated and CCL-BP
samples are shown in FIG. 9.

[0050] FIG. 9a shows the tensile measurement of CCL-MWCNT-BP samples. The
randomly oriented CCL-MWNCT-BP revealed that the average tensile strength
was 150 MPa, which was two times stronger than that of the pristine
samples. The Young's moduli of the randomly oriented pristine MWNT-BP and
CCL-MWCNT-BP were 1.04 GPa and 10.18 GPa, respectively. The average
elongation to break of both the randomly oriented and aligned pristine
MWCNT-BPs was 20.0%, which was seven times higher than that for the
CCL-MWCNT-BP samples. These results indicate an improvement in load
transfer and less nanotube slippage after the cross-link reaction. The
pristine BPs showed a noticeable plateau in the stress-strain curves and
low mechanical properties due to CNT slipping and limited inter-tube
interactions and load transfer. Cross-linking of CNTs was an effective
approach to effectively eliminate sliding between the CNTs. FIG. 9b shows
the tensile properties of the aligned CCL-MWCNT-BPs having improved
mechanical and electrical properties. The tensile strength was 220 MPa,
two times stronger than the tensile strength of pristine aligned MWCNT
films. The Young's moduli of the pristine aligned BP and CCL-MWCNT-BPs
were 1.91 GPa and 8.8 GPa, respectively. FIG. 9c shows the tensile
properties of SWCNT-BP samples. The tensile strength of the CCL-SWCNT-BP
was 65 MPa, which was four times stronger than the tensile strength of
the pristine sample. The Young's moduli of the pristine and CCL-SWCNT-BP
samples were 2.02 GPa and 8.6 GPa, respectively. The cross-links led to a
seven fold increase in Young's modulus and a four fold increase in
tensile strength for the CCL-SWNT-BP samples.

[0051] Modifications and variations of the methods and devices described
herein will be obvious to those skilled in the art from the foregoing
detailed description. Such modifications and variations are intended to
come within the scope of the appended claims.